This invention relates to an integration sphere type turbidimeter capable of measuring the turbidity of a sample liquid accurately by the optical diffusion method even when the sample liquid contains dispersed particles of an extremely small particle diameter in an extremely low concentration.
Generally, the conventional integration sphere type turbidimeter comprises an integration sphere, a sample cell disposed outside the integration sphere and adapted to hold a given sample liquid therein, an optical device for passing a collimated beam of light through the sample cell into the integration sphere, and a light receiving element disposed on the integration sphere and adapted to pick up diffused light resulting from the impingement of a part of the collimated beam of light upon the dispersed particles of the sample liquid in the sample cell and, further comprises another light receiving element to pick up the part of the collimated beam of light which passes through the sample cell, whereby the turbidity of the sample liquid is determined by comparing the amounts of light picked up by the two light receiving elements mentioned above (British Pat. No. 1,089,848 and U.S. Pat. No. 4,165,179, for example). In the conventional turbidimeter wherein the sample cell is disposed outside of the integration sphere and the collimated beam of light passes through the sample cell and then into the sphere, the maximum amount of diffused light that can possibly enter the integration sphere is that falling within the range of 180.degree. of the sample cell on the integration sphere side, this maximum being realized when the sample cell is in direct contact with the integration sphere. Enlargement of the sample cell does not help to increase the amount of diffusee light received by the integration sphere since the turbidimeter can measure turbidity only in the portion penetrated by the incident collimated beam of light and, consequently, the region which produces the light diffusion is not a function of cell size. As a result, the conventional turbidimeter has been unable to provide effective measurement of turbidity where the sample liquid contains dispersed particles having a diameter on the order of 1.mu. at a low concentration of less than 500 ppb, for example.
As a way of overcoming this problem, it might be thought that the extreme smallness of the amount of diffused light picked up by the light receiving element could be compensated for by greatly increasing the gain of the amplifier system connected to the output terminal of the photo-electric conversion element which picks up the diffused light. This would be a false assumption, however, since when the gain is increased past a certain level and the input is of extremely small magnitude, there ensure problems relating to linearity and amplification ratio. In the first place, the relationship between light received and electrical output of the photoelectric conversion element has poor linearity when the amount of light picked up is very small. An attempt to eliminate this difficulty electrically by the adoption of a compensatory circuit would call for enormous amounts of effort and expense and would add greatly to the complexity of the measuring system.
The needs of industry on the other hand are no longer met by the actual performance of conventional turbidimeters. As a consequence, there has arisen a need to develop a turbidimeter which is capable of measuring, accurately to within a graduation as fine as 50 ppb, extremely low degrees of turbidity such as in sample liquids having dispersed therein very minute particles having a particle diameter of less than 0.5.mu., for example.
The object of this invention is to provide a turbidimeter capable of accurately measuring extremely low degrees of turbidity.